1. Field of the Invention
The present invention relates to a surface acoustic wave filter and more particularly to a surface acoustic wave filter in which an input IDT (Interdigital transducer) and an output IDT are arranged in a direction perpendicular to an oscillation direction of a surface acoustic wave.
2. Description of the Related Art
Today, in accordance with scale reduction of communication apparatus such as a portable telephone, scale reduction of electronic components used in the apparatus is strongly desired. Especially, a surface acoustic wave filter used in a duplexer is prevalently used in portable telephones because it is advantageous in terms of its sharp frequency characteristics and scale reduction of the apparatus.
A surface acoustic wave filter includes an input IDT for receiving an electric signal to oscillate a surface acoustic wave and an output IDT for receiving the oscillated surface acoustic wave to output an electric signal. As the input IDT and the output IDT, interdigital electrodes (comb-like electrodes) obtained by a combination of elongated electrode fingers are used.
Also, for surface acoustic wave filters requiring sharp characteristics such as IF filters, interdigital transducers with weighting, such as apodized and withdrawal, have been used both as the input IDT and the output IDT. The amount of weighting the electrodes such as the number of electrode pairs is usually determined by filter characteristics such as a bandpass width. Accordingly as the required filter characteristics are sharper, the amount of weighting the electrodes, i.e. the number of electrode pairs, increases, so that the size of the interdigital transducers increases and hence the filter size itself in the oscillation direction of the surface acoustic wave increases.
FIGS. 18 and 19 show constructions of conventional surface acoustic wave filters. FIG. 18 shows a surface acoustic wave filter obtained by a combination of an apodized IDT 31 and a uniform IDT 32. FIG. 19 shows a surface acoustic wave filter obtained by a combination of an apodized IDT 31 and a withdrawal IDT 33.
Assuming that the apodized IDT 31 is the input IDT, a surface acoustic wave is oscillated between its electrode fingers in the right-and-left direction of the paper sheet and propagates in the right and left directions from the apodized IDT 31. In FIGS. 18 and 19, the surface acoustic wave propagating in the right direction from the apodized IDT 31 enters the output IDT which is the uniform IDT 32 or the withdrawal IDT 33.
In the conventional constructions shown in FIGS. 18 and 19, it is required that the number of electrode pairs in the apodized IDT 31 is about 1000 and the number of electrode pairs in the withdrawal IDT 33 or the uniform IDT 32 is about 400 in order to satisfy the characteristics required as an IF filter, although they depend on the specification of the filter.
Also, Japanese Unexamined Patent Publication No. HEI 9 (1997)-153765 discloses a SAW filter comprising four electrodes forming a multistrip coupler which are each bent in the shape of a crank and mutually cross at the intermediate section through insulating films, an input transducer and an output transducer, the input transducer and the output transducer being respectively arranged above and under the crossing sections of the four electrodes on the left or right side of the multistrip coupler.
However, in the case where an apodized IDT having as many as 1000 pairs of electrode fingers is used, the filter size in the oscillation direction of the surface acoustic wave increases considerably, rendering it impossible to meet the needs for further size reduction.
Also, in Japanese Unexamined Patent Publication No. HEI 9(1997)-153765, the apodized IDTs used as the input IDT and the output IDT require about 1000 pairs of electrode fingers, so that a further size reduction is difficult. Moreover, since the filter is provided with a structure in which a multi-strip coupler crosses over in three dimensions, the fabrication process has been difficult.
Further, in order to reduce the size of the surface acoustic wave filter, the number of electrode pairs (i.e. the amount of weighting the electrodes) must be reduced. However, desired frequency characteristics such as required in IF filters cannot be obtained if the amount of weighting the electrodes only is reduced in a simple manner.